1. Field of the Invention
The invention relates in general to a method of optical proximity correction (OPC). More particularly, this invention relates to a method of optical proximity correction using both binary mask and phase shift mask (PSM).
2. Description of the Related Art
Currently, two types of photomasks including the binary mask and the phase shift mask are frequently used for the photolithography process. The phase shift mask provides a better resolution than the binary mask, while the phase shift mask, though provides a better fidelity, is more difficult to fabricate and consumes a higher fabrication cost.
To prevent the deviation of critical dimension while transferring a pattern of a photomask, an optical proximity correction is used to obtain the fidelity of the resultant pattern. An original pattern to be transferred onto a wafer is provided. Being calculated by a computer and a related software, the data of the original pattern is modified to obtained a corrected pattern saved in the computer. The corrected pattern is then fabricated on a photomask. By performing processes such as photolithography and etching, the original pattern is obtained on the wafer.
To optimize the fidelity of a pattern transferred onto the wafer, it is a trend to utilize a mixed mode for both the phase shift mask and the binary mask. However, the optical characteristics of the phase shift mask are very much different from those of the binary mask. The difference in optical characteristics cause a great difficulty in practical application.
FIG. 1 shows a relationship between a critical dimension of a layout on a photomask and a critical dimension of a pattern transferred onto the wafer. The curve a reflects the characteristic for using the phase shift mask, while the curve b illustrates the characteristic for using the binary mask. When the critical dimension reduces to a certain value, the slope of the curve a is increased as the critical dimension decreases. On the contrary, the slope of the curve b is decreased as the increase of the critical dimension. The contradictory behaviors of the curves a and b causes a great difficulty of implementing both the phase shift mask and the binary mask at the same time.